That's an agreed on contract. Nothing funny about that. If you can move 10 grand worth of prime rib and put your money where your mouth is, you have earned the reduced price.
A kickback is when you spend, say $4,000 a month with the produce guy, and he slips you an envelope with $200 cash at the end of the month.
The employer is out $200, and it shows up as over priced goods. The employer would be better off offering a 5% bonus on food cost--at least the Chef/purchaser would sweat for it.....